Device for locating objects that emit electromagnetic signals

ABSTRACT

A locator such as a ground penetration probe (24) has spaced antennae (21,22,23) therein which detect electromagnetic signals from an object (26) such as a buried cable. By analyzing the electromagnetic signals using a suitable processor (25) it is possible to determine the separation of the locator and object (26), both in terms of the direction (X) corresponding to the spacing of the antennae (21,22,23) and the perpendicular direction (Y) to the object (26). This then permits a display to be generated showing visually the separation of the locator and the object (26). If the locator incorporates a tilt sensor, the processor (25) can then compensate for tilting of the locator, and determine the vertical and horizontal separation of the locator and the object (26). A confidence measurement may be obtained by measuring the separation of the locator and object (26) at one position, predicting the separation of the locator and object (26) at a second position, and comparing the predicted and measured separations.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a locator for locating a conductiveobject. It is particularly, but not exclusively, concerned with alocator for locating an underground conductor such as a buried cable orpipe.

2. Summary of the Prior Art

The proliferation of networks of buried cables and pipes for manydifferent utilities (electricity, gas, telecommunications, etc) hasmeant that any excavation of the ground is likely to be in the vicinityof a buried cable or pipe, and such excavation involves a risk of damageor interference to the buried cable or pipe, unless the location of thatburied cable or pipe is precisely known.

In particular, the growth in the use of fibre-optic communicationsystems for telephones, cable television, etc has significantlyincreased the problems associated with excavation. Such fibre-opticcommunication systems have a much higher communication capacity thanmetallic conductors, but the costs consequent on damage or interferenceto the fibre-optic communication system are significant. Moreover, ifdamage occurs, it is more difficult to repair a fibre-optic connectionthan it is to repair a metallic connection. For this reason, ownersand/or operators of fibre-optic communication systems normally requirethat, before any excavation can occur in the vicinity thereof, thelocation of the fibre-optic connection should be determined precisely,both by suitable location system and by visual inspection. In practice,this means that an initial excavation needs to be made to permit theofficial inspection of the cable, before any more extensive excavationcan be carried out in the vicinity. Moreover, each preliminary locationand excavation to enable the fibre-optic connection to be inspected mustbe repeated along the length of the fibre-optic connection, and thisrequires a significant amount of time and effort.

One type of conventional locator detects alternating fields from signalcurrents in a conductor, by means of a suitable antenna assemblyincorporated in a hand-held receiver. Such an arrangement is applicableto fibre-optic connections because such connections normally have ametal sheath for protection purposes, and a signal can be applied tothat metal sheath and detected.

In such conventional systems, a user carries the receiver and repeatedlymakes measurements adjacent the target conductor until the receiverindicates that the conductor is present. Then, in order to obtain thevisual inspection referred to previously, an excavation is made at thesite determined by the locator, until the pipe, cable or fibre-opticconnection is exposed.

SUMMARY OF THE INVENTION

In a first aspect of the present invention, a locator is provided withat least two antennae of known separation, each antenna being able tomeasure electro-magnetic field components in the direction of theseparation of the two antennas, and perpendicular to that direction tothe object. Suitable processing means is then provided to deriveco-ordinate information defining the direction and separation, of thelocator relative to the conductor which generates the electro-magneticfields. In the following specification, the direction corresponding tothe separation of the antennas will be referred to as the X direction,and the perpendicular direction which intersects with the object will bereferred to as the Y direction. Thus, the present invention derives Xand Y co-ordinate information of the object relative to the locator.

Preferably, the locator is in the form of a ground penetration probe.Then, as the probe penetrates the ground, the antennae detectelectromagnetic signals from a conductor of an underground object to belocated, and can determine the position of the probe relative to theunderground object. Thus, the probe can be driven into the groundtowards the underground object and the user will be provided withinformation which indicates the separation of the probe from the object,to enable the probe to be brought into close proximity to theunderground object without the risk of the probe damaging theunderground object due to forceful impact.

In order to determine the separation of the locator from the object, atleast two antennas must be provided at spaced-apart locations of thelocator. The difference in electromagnetic fields detected by the twoantennas then enable the distance from the locator to the object to thecalculated, to generate suitable information to the operator.Preferably, three or more antennas are provided, as this then permitsmore accurate measurements to be made as the locator approaches theunderground object by choice of sensing antennas utilized. This isparticularly useful when the locator is a ground penetration probe, sothe accuracy of location is improved as the probe approaches closeproximity to the underground object.

At least in theory, the position of the object relative to the locatorcan be determined by assessment of the electromagnetic fields at the twoantennae, and by calculations using simple trigonometry. However, inpractice, it is likely that there will be asymmetries in theelectromagnetic field generated by the conductor, for example because ofthe presence of other adjacent conductors, and therefore it ispreferable that the processing means is provided with suitablecompensation for such errors. Moreover, although it is possible to haveantennae with coils of common centres, it is often more practical tohave coils with displaced centers, in which case suitable compensationmust be provided for this as well.

Although it is possible for each antenna to have two-axis coils, it ispreferable that three-axis coils are provided since this enables furtherinformation to be derived which enables the inclination of the directionof extension of the object relative to the Y direction to be determined.

In a further development, the locator is provided with tilt sensingmeans which enables the inclination of the locator, and hence the Xdirection, to be determined relative to the vertical. The processingmeans then makes use of this tilt information, and it is then possibleto derive a determination of the location of the object, in terms of itsvertical and horizontal separation from the locator, independent of theorientation of the locator. This is important since it is not easy forany operator to ensure that the locator is held absolutely vertically.

This aspect of the present invention is applicable to the location ofany conductive object from which an electromagnetic signal can betransmitted. As has previously been mentioned, the present invention isprimarily concerned with the location of a buried fibre-opticconnection, by applying signals to a metal sheath of such a connectionand detecting the electromagnetic fields generated therefrom, but thepresent invention is not limited to this field of application.

As has been mentioned above, a locator with three or more antennae ispreferred as it enables more accurate measurements to be made as thelocator approaches the object. Since the position of the object relativeto the locator can be determined by any pair of antennae, it is possiblefor the choice of antenna pair to be changed in dependence on theseparation of the locator and the object. This switching between antennapairs, when carried out automatically, represents a second independentaspect of the present invention.

Also as mentioned above, the locator of the present invention may havethree-axis antennae which enable the relative position of the objectlocated to be determined, but also the angle of inclination of thedirection of elongation of the object relative to the Y direction. As aresult, it then becomes possible to predict the magnetic field whichwill be generated by the object at a position close to, but separatedfrom, the current position of the locator. If the locator is then movedto that position, and the field measured, the correspondence between themeasured field and the predicted field gives a measure of the confidencelevel of the measurement. This operation may be carried out by movingthe locator to a predetermined lateral displacement. However, a similareffect can be achieved by changing the inclination of the locatorrelative to the vertical. Since that change in inclination can bemeasured by a tilt sensor, it is then not necessary to move the locatorby a predetermined amount, because the tilt sensor can then determineany change in inclination. This simplifies the actions needed by theoperator, since the operator merely needs to change the angle of thelocator relative to the vertical in order to make a measurement of thedegree of confidence in the location of the object. This way ofobtaining a confidence measurement, by moving the locator, thereforerepresents a third independent aspect of the present invention.

If the locator is a ground penetration probe, it will be normal for theprobe to be a drilling device which is driven into the ground at theapproximate location of the underground object by the operator. Usingthe information from the antennae, the locator can bring the probe hipinto close proximity to the underground object, since the operator isprovided with information relating to the separation therebetween, andcan control the movement of the probe appropriately.

The probe may have an outer sheath from which the rest of the probe canbe removed. Then, the probe together with the sheath is inserted intothe ground until the buried object is reached, and then the rest of theprobe removed from the sheath to permit access for inspection ormaintenance. For example, an endoscope may be inserted into the sheathto give visual information about the underground object. The sheath maybe left in place for subsequent access, or as a marker.

Alternatively, the probe leaves a hollow since this permits visualinspection of the underground object by endoscope or other inspectionmeans inserted into the space left by the probe once the probe has beenbrought into close proximity with the underground object and removedfrom the ground.

Where the probe is a drilling device, the information from the antennaemay be used to control the drilling force. For example, when the probeis a long way from the underground object, the drilling force can belarge so that the probe moves rapidly towards the underground object. Asthe probe approaches the underground object, and to prevent forcefulimpact, the drilling force may be reduced so that the drilling force isminimal as the probe reaches the immediate vicinity of the undergroundobject.

In the aspects of the present invention discussed above, antennae detectsignals from the object. Normally, signals are applied to a conductor ofthat object from a separate transmitter. However, if such a system isapplied to the location of underground objects using a groundpenetration probe, there is a risk that there may be other objects whichcould be hit by the probe as the probe penetrates the ground. Fornonmetallic objects, this problem can be resolved by providing othersensing means, e.g. radar or accelerometer, to enable probe movement tobe halted on or just prior to impact with such an object. That othersensing means may also be used for specific sensing and/or locatingtasks connected with solid objects. If the underground object has aconductor, however, it is possible for the probe to have a transmittertherein which transmits signals which induce further signals in theunderground object, which further signals can then be detected. This maybe useful, for example, where there is a dense network of differentconductors at the site to be investigated.

Alternatively, the probe may detect and locate the underground object onthe basis of signals already present on the object, for example 50Hz or60Hz mains power or radio signals.

In a further development, the object may have one or more devicesthereon which are able to transmit a predetermined signal. Suchtransmitters are known, in themselves from e.g. from animal husbandry,in which they are referred to as RFID systems. A locator is providedwith means for detecting such devices, so further identification of theobject can be achieved.

In the present invention, the relationship in space between the objectand the locator is determined. Hence, with the present invention, itbecomes possible to generate a visual display showing its spatialrelationships thereof, rather than e.g. by a audible signal which variesits frequency. Such a generation of a visual display is therefore afourth aspect of the present invention.

In this fourth aspect, the operator of the locator e.g. a groundpenetration probe may make use of the visual display in order to controlthe movement of the locator. Where the locator is a ground penetrationprobe e.g. a drill, the user may alter the display and change thedirection of movement of the drill so that the drill approaches theunderground object. The user can therefore ensure that the drill is atall times targeted towards the underground object. Since the user ispresented with a visual display of the separation (and it may bepossible for that display to change in magnification as the probeapproaches the object), it would be possible for the user to stop themovement of the probe very close to the object. However, it ispreferable, as previously described, that there is automatic control ofthe speed of movement of the probe in accordance with the second aspectof the present invention, to reduce the risk of accidental damage to theobject.

The visual display of this fourth aspect is preferably a head-up displaywhich presents the spatial relationship of the probe and the undergroundobject in the normal line of sight of the user. This has the advantagethat the user may simultaneously view that display and at the same timesee the movement of the probe.

Indeed, such a head-up display may be applied to other types of locatorsand is thus a fifth independent, aspect of the present invention.

It is often the case that a site to be investigated has a plurality ofadjacent objects. For example, the need for cables of differentutilities to follow similar routes means that it is often necessary forthe operator of a ground penetration probe to be aware of all theunderground objects at the particular site. If all the undergroundobjects generated the same electromagnetic signals, the antennae of thelocator would simply record the composite field generated. However, ifthe different signals are applied to the different underground objects,for example signals of different frequencies, then it is possible forthe locator to distinguish between the different objects by suitablemodulation of the signals received. Hence, by suitable analysis, it ispossible to determine the separation of the locator from each of theunderground objects, and for each underground object to be displayed onthe display. Hence, the operator is then presented with informationshowing the location of all the underground objects at the site. This isimportant, for example, in ensuring that the ground penetration probeapproaches only the underground object of particular interest, andavoids other underground objects. Because the operator is presented witha visual display showing the relevant positions of the objects, becausetheir positions relative to the locator are known, it is possible forthe operator to bring a ground penetration probe into close proximitywith one underground object, while avoiding contact with otherunderground objects at the site.

As an alternative to applying different signals to the undergroundobjects, which is not always practical, the underground objects at thesite may be distinguished if they each carry active markers as describedpreviously, with each active marker generating a coded signal whichidentifies the particular underground object. Since the locator can thendetect information which identifies the number of underground objects atthe site, the locator can then resolve signals it receives intodifferent components corresponding to the different objects.

As has previously been mentioned, the ground penetration probe is driveninto the ground towards the underground object. For very soft ground,this could be done simply by the user applying force to the groundpenetration probe, but preferably a mechanical drive is provided.

According to a sixth aspect of the present invention, that drive isprovided by contra-rotating masses. If two masses are driven at angularvelocities, about a pivot point, there will be a net force on the pivotpoint which is determined by the phase between the two masses, and bythe masses themselves. By suitable arrangement of the masses, and thephase therebetween, it is possible to arrange for the variation in forceto be such that the downward force has a magnitude which is greater thanany upward force, even if the time-average is zero. Then, bearing inmind that the penetration probe must overcome friction with the groundin order to move, the forces can be such that the downward force issufficient to drive the ground penetration probe into the ground but theupward forces do not overcome friction sufficiently to drive the groundpenetration upwards to the same extent, so that there is a net downwardmovement. Similarly, by altering the phase of rotation of the masses, itis possible to re-arrange the system so that the upward forces exceedthe downward forces in magnitude, so that the ground penetration probewill be driven out of the ground. Preferably, two such pairs of massesare used to cancel-out lateral forces.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described in detail, byway of example, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view from the side of the relationship between aground penetration probe according to the present invention and anunderground object;

FIG. 2 corresponds to the ground penetration of FIG. 1, but viewed fromabove;

FIG. 3 is a schematic view showing measurement of the relationshipbetween the ground penetration probe of FIG. 1 and two undergroundobjects;

FIG. 4 shows the movement of the ground penetration of FIG. 1 relativeto the underground objects;

FIG. 5A shows practical embodiment of the ground penetration probe ofFIG. 1;

FIG. 5B shows in more detail the handle of the embodiment;

FIG. 5C shows in more detail the arrangement of coils in an antenna inthe embodiment of FIG. 5A;

FIG. 6 shows a signal processing system for use with the groundpenetration probe of FIG. 1;

FIG. 7 shows a schematic view of a detailed embodiment of a locatorembodying the present invention;

FIGS. 8 to 10 are views of a tool head in the embodiment of FIG. 7, FIG.8 being a transverse sectional view, FIG. 9 being a sectional view inplan, and FIG. 10 being a side view, partially in section;

FIGS. 11a and 11b illustrate the movement of masses in the eccentrics inthe tool head of FIGS. 8 to 10;

FIG. 12 is a graph showing the acceleration characteristics that arethen achieved in the tool head of FIGS. 8 to 10; and

FIG. 13 is a schematic view of the display generated by the head-updisplay of the location of FIG. 7;

FIG. 14 shows a portable locator being a further embodiment of thepresent invention;

FIG. 15 shows in more detail the locator and display unit of theembodiment of FIG. 14; and

FIG. 16 shows a typical display generated by the display unit of FIG.15.

DETAILED DESCRIPTION

As has previously been described, the various aspects of the presentinvention discussed above are based on the detection of the separationof a locator and an object. The present invention is particularly, butnot exclusively concerned with the detection of the separation of aground penetration probe and an underground object. The basic principlesunderlying such detection will now be discussed with reference to FIGS.1 to 4

Referring first to FIG. 1, a ground penetration probe 24 has threeantennae 21, 22, 23 thereon. The antennae detect electromagnetic fieldsgenerated at an underground object 26, such as an underground conductorcarrying current. The structure of antennae capable of detecting suchradiation is, in itself, known and will not be discussed further.

One of the antennas 21 is located at, or a known distance from, the tipof the ground penetration probe 24, and the other two detectors 22, 23are at known separations S1 and S2 respectively along the groundpenetration probe 24 from the first antennas 21. The ground penetrationprobe 24 ensures that the antennas 21 to 23 have a known separation andfixed orientation so that signals detected thereby can be processed.

When the ground penetration probe 24 is moved into the ground proximatean underground object 26, which underground object carries an AC signal,electromagnetic fields will be detected by the antennas 21 and 22enabling position vectors V₁ and V₂ to be calculated, and permitting theseparation of the ground penetration probe 24 from the undergroundobject 26 to be determined e.g. in terms of X and Y positioncoordinates. Such calculation may be carried out by a suitable processor25 connected to the antennas 21 to 23 via the probe 24.

Thus, if:

B is the total field detected at antenna 21,

T is the total field detected at antenna 22,

Bh is the horizontal component at antenna 21,

Th is the horizontal component of the field at antenna 22,

Bv is the vertical component of the field at antenna 22,

then the X and Y position co-ordinates are given by: ##EQU1##

Since the X and Y co-ordinates can be calculated the position of vectorsV₁ and V₂ can be calculated. These equations hold true for the simplecase of a single conductor, and are given by way of example.

Furthermore, if the antennas 21 to 23 are each based on a three-axisorthogonal aerial system, it is also possible to obtain a thirdorientation vector V₃ as shown in FIG. 2, to derive the angle θ beingthe plan rotation of the ground penetration probe 24 relative to theunderground conductor 26. Therefore, by knowing the X and Y coordinatesand the angle θ, a user can be presented with information indicating theseparation of the ground penetration probe 24, and in particular the tipthereof, from the underground object 26. The user may then direct themovement of the ground penetration probe 24 to reduce that separation,e.g. to bring the tip of the ground penetration probe 24 immediatelyadjacent the underground object 26.

From the above description, it can be seen that only two antennas 21, 22are needed in order to determine the separation of the tip of the groundpenetration probe 24 and the underground object 26. However, as the tipof the ground penetration probe 24 approaches the underground object, itis possible that the determination of the separation makes use of thethird antennas 23, as shown in FIG. 3. Measurements made on the firstand third antennas 21, 23 then offer a more accurate measurement of theseparation of the tip of the probe 24 from the underground object 26when that separation is small.

Furthermore, FIG. 3 also shows that, as the tip of the probe 24approaches the underground object 26, the effect of other generators ofelectromagnetic fields, e.g. object 27 is reduced since the antennae 23is used only when the tip of the probe is close to the undergroundobject 26. The change in ratio of antenna separation distance tounderground object distance will reduce the relative effect of signalsfrom object 27 enabling it to be identified as being other than thedesired underground object 26. Furthermore, the known proximity of theunderground object 26 also enables signal current direction andamplitude to be determined with greater confidence than if only widerspaced antennas are used.

As has previously been mentioned, the processor 25 determines theseparation of the tip of the probe 24 from the underground object 26.Therefore, if the angle of orientation of the probe 24 is changed, thisshould not affect separation of the tip of the probe 24 from theunderground object 26. Thus, if the ground penetration probe 24 ispivoted through an angle o from position A to B in FIG. 4, themeasurement of the position of the underground object 26 should notchange. Similarly, if the probe 24 is moved laterally, e.g. to theposition C shown in FIG. 4, then the lateral movement from position A toC corresponds to the change in position of the object 26 relative to theprobe 24. In other words, the object 26 should maintain its absoluteposition.

One possible impediment to the accuracy of derived positionalinformation is tilt of the locator axis from true vertical. This may becountered by incorporation of a 2-axis tilt sensor of any suitabledesign giving electrical data corresponding to angular deviation fromthe vertical gravitational axis, from which the true location of thetarget object can be computer, by appropriate compensation of thelocator data.

A practical limitation of the locator system as described with referenceto FIGS. 1 and 4 is that it uses antenna arrays having common centresand vertical axes. This is difficult to achieve in practice, even withspherical cores common to orthogonal coils; with separate solenoidalcoils and cores, or corresponding magnetic field sensor arrays of othertypes, achievement of common centrality to a high degree of accuracy isimpracticable. It is therefore preferable to incorporate mathematicalcompensation for the deviations from centrality of the coils of theantennae.

Another limitation to the accuracy of positional data from multi-antennalocators is perturbation of the magnetic field. It is thereforedesirable to incorporate mathematical compensation for fieldperturbation of the sensed field, to increase the location accuracy, orestablish a confidence level for the data derived.

The mathematical compensations needed to take into account deviationsfrom the centrality of the close of the antennae and for fieldperturbation means that the equations referred to earlier to determine Xand Y cannot be used. Instead, more complex equations are needed, aswill now be described. Referring to FIG. 1, but assuming that the groundpenetration probe 24 is inclined by an angle φ, then: ##EQU2##

It is then possible to define the angle α to the cable from the antenna21 and angle β to the cable from the antenna 22. Then the field in thevertical coil of antenna 22 is: ##EQU3## where I is the cable current.

Similarly the field in one of the horizontal coils of coils of antenna22 is: ##EQU4## and the field in the other horizontal coil of antenna 22perpendicular to the first horizontal coil is: ##EQU5##

Similarly, the field in the vertical coil of antenna 21 is: ##EQU6##

The field in one of the horizontal coils of antenna 21 is: ##EQU7## andthe field in the other of the horizontal coils of antenna 21 is:##EQU8##

Since the signals measured by the antennae 21,22 relate directly to thefields, those signals can be used directly in the calculations of cableposition.

Thus, the total field T at the antenna 22 is: ##EQU9## and the totalfield at the antenna 21 is: ##EQU10##

V₁ is then the solution to a quadratic equation, and hence there are twopossible solutions: ##EQU11##

The choice of which of solutions V₁ (1) and V₂ (2) is correct depends onthe sign of the horizontal field at antenna 22: ##EQU12##

The above calculations then need to be corrected if the axes of twohorizontal coils do not intersect.

Define the sum H₂ of the horizontal fields in one direction at the twoantennae 21,22

    H.sub.2 =B.sub.2 +T.sub.2

Similarly, define the sum H₃ of the horizontal fields in theperpendicular direction at the two antennae 21,22

    H.sub.3 =B.sub.3 +T.sub.3

Then θ is the arctan of the ratio of H₂ and H₃.

Then the correction to X needed if the axes of the two coils aredisplaced by a distance d is:

    Xcorr=X+dsinθ

These differences are then corrected for the probe tilt to give thevertical distance D from the lower sensor 21 to the cable 26 and thehorizontal distance H from the lower sensor 21 to the cable 26

    D=Xcosφ+Ysinφ

    H=Xsinφ+Ycosφ

FIGS. 5A to 5C show in more detail a practical embodiment of the groundpenetration probe being an embodiment of the present invention.

In this embodiment, three antennae 150A, 150B and 150C are containedwithin the tubular housing 160, so that the separation between theantennae 150A, 150B and 150C is fixed. The antennae thus correspond tothe antennae 21 to 23 in FIGS. 1 to 4.

Each antenna 150A, 150B, 150C, nominally horizontal coils 151 and 152 atabout 90° to each other, and a nominally vertical coil 153. Whendesigned for incorporation into the tubular housing 160 of smalldiameter such as a ground penetrating probe, the coils 151,152,153 haveinevitably a low ratio of length to diameter, so that small dimensionalvariations result in significant departures from true perpendicularity.In addition, the vertical separation of individual coils in each antenna150A, 150B, 150C means that the three axes of measurement are not oncommon centres, although the vertical separation of the three antennaefrom each other can be controlled quite accurately by the supportstructure 154. This is typically made from rigid plastic, incorporatingappropriately positioned slots to accommodate the coils, as in detailFIG. 5C. The support structure 154 is firmly located within the outertubular housing 160. This tube may have a handle 156 when used as aportable locator above the ground, or it may be the ground penetratingtube in drilling applications. The tubular housing 160 provides forelectrical interconnections 157 (see FIG. 5B) and can also accommodatecircuit boards and other sensing means as required, e.g. at 158, andtilt sensor e.g. at 159.

One possible sensor that could be used as a sensing means 158 in FIG. 5is a sensor designed to interrogate markers of the transponder type.Technology of such markers is known as such, and the sensor interrogatesthe marker in a way which identifies the marker. The marker incorporatesa transponder tuned to a specific carrier frequency. The sensor 158 inthe locator then transmits energy to the transponder which is convertedby induction in the transponder, using a tuned pick-up coil, to power are-transmitting circuit of the marker. The output of the re-transmittingcircuit is at the carrier frequency, but is modulated by appropriatemeans within the marker to encode data identifying the marker, and hencethe object to which the marker is attached. The modulation is decoded bya receiving circuit of the sensor 158. Once the marker has beenidentified, an appropriate display may be generated as will be describedin more detail later.

It was mentioned above that, if the probe 24 in FIGS. 1 to 4 is movedlaterally, or pivoted, the measurement of the position of theunderground object 26 should not change. This enables the degree ofconfidence of the location of the underground object 26 to be measuredeasily. Referring to FIG. 4, suppose that a measurement is made of theposition of the underground object 26 at position A, and the groundpenetration probe 24 is then moved by a known amount to position C. Ifthe processing means of the locator is aware of the lateral separationof positions A and C, the measurement at position A should enable aprediction to be made of the result of the measurement at position C.Since the location of the object 26 does not change, since theseparation of positions A and C is known, the fields measured by theantennae 21,22 at position C is predictable. Therefore, if the probe 24is moved to position C, and measurements made, a comparison can be madebetween the actual measurements at position C and the predictedmeasurements from the measurement at A. If these coincide, there is ahigh degree of confidence that the object 26 has been locatedaccurately. If, however, there is a substantial divergence between thepredicted measurements at C and the actual measurements at thatposition, the accuracy of location of the object 26 is questionable, sothere is then a low degree of confidence. This procedure requires thelateral separation of positions A and C to be known. The user of theground penetration probe 24 must therefore move that probe 24 by thatknown amount. This may be inconvenient, or difficult to achievepractically. However, a similar effect can be achieved by pivoting theprobe 24 through the angle φ from position A to B in FIG. 4. Again,assuming that measurements are made in position A, it is possible topredict the results of the measurements made in position B, assumingthat the angle is known. It is then possible to measure angles by usingthe tilt sensor described with reference to FIG. 5, and that measurementof the angle can be applied to the prediction. As a result, it is notnecessary for the user to move the probe 24 by a known amount, since theangle of tilt can be measured independently by the tilt sensor. As aresult, the user determines the position of the object 26 at theposition A, tilts the probe 24 by any suitable amount φ and theprocessing means then calculates the predicted measurements based on themeasured angle of tilt φ, and at the same time determines the actualmeasurements at that angle of tilt φ. This permitting the actual andpredicted measurements to be compared to give a measure of the degree ofconfidence of the measurement.

FIG. 6 shows the signal processing system in each antennae 21 to 23. Thesignal IP from the corresponding antennae 21 to 23 is passed via anamplifier 30 and a low pass filter 31 to an analog-to-digital converter32. The low pass filter 31 eliminates unwanted frequencies in thesignal, so that the signal generated by the analog-to-digital converter32 may be passed to a digital signal processor 33 to permit amplitudeand phase signals to be generated. FIG. 6 also shows that the digitalsignal processor 33 can be used to control gain of the amplifier 30.Amplitude and phase signals thus generated from each of the antennae 21to 23 are passed to a microcomputer 34 which calculates the X, Y and θmeasurements defining the relationship between the probe 24 and theunderground object 26, and may be stored in a suitable recording system35 and/or used to generate a display 36 (e.g. a head-up display).

Another embodiment of the present invention is illustrated in FIG. 7. Inthis embodiment, the ground penetration probe is a drilling bar 40 whichis driven from a tool head 41. The drilling bar 40 contains the antennas21 to 23 described previously, but those antennas are contained withinthe drilling bar 40 and are therefore not visible in FIG. 7. The toolhead 41 receives power via a base station 42 and power line 43, the basestation 42 either containing its own power source or being powered froma separate power supply 44. That separate power supply 44 isvehicle-mounted in the embodiment of FIG. 7, so that the whole system istransportable. The base station 42 also contains the processor 25described previously, with the processor receiving signals from theantennas in the drilling bar 40 via a line 45 which is also connected toa pack 46 on the belt of the user. Thus, the processor may send signalsvia the line 45 and the pack 46 to a head-up display 47 to permit theuser to obtain an immediate visual indication of the separation of thetip of the drilling bar 40 from the underground object.

The base unit 42 may be equipped with suitable memories to store dataderived from the antennae, to provide a more permanent record of themovement of the drilling bar 40 relative to the underground object. Ifthe system is also arranged to provide warning of other sources ofelectromagnetic fields, or also a warning of close proximity of thedrilling bar 40 to the underground object, these may be passed to thebase unit 42 via the line 45 and the pack 46 to earphones 48 for theuser.

The powering of the drilling bar 40 by the tool head 41 will now bedescribed with reference to FIGS. 8 to 10.

The sectional view of FIG. 8 shows that the drilling bar 40 is clampedby a collet 50 to a hollow shaft 51 which is rigidly fixed to the casing52 of the tool head 41. Handles 53 are resiliently attached to thecasing 52 via torsion assemblies 54. In FIG. 8, the handle 53 andtorsion assembly 54 on the left-hand side are shown in section, whilstthe handle 53 and torsion assembly 54 on the right-hand side are shownfrom the exterior thereof. The torsion assemblies 54 reduce vibrationpassed to the user from the drilling head 41. A control switch 55 ispreferably provided adjacent one of the handles 53 to permit the user tocontrol the action of the drilling bar.

As can been seen from FIG. 9, which shows the tool head 41 in plan view,the switch 55 is connected via a wire 56 to a control unit 57 whichcontrols a motor 58. The motor 58 is also shown in the side view of FIG.10, which illustrates the separate casing 59 for the motor 58, and alsothe right-angle gear box 60. That gear box 60 connects the motor 58 to ashaft 61 which is coupled by a gear train 62 to a pair ofcontra-rotating shafts 63. Those shafts 63 are each rigidly coupled toan eccentric 64 and by lost-motion to a phased eccentric 65. The purposeof the use of lost-motion is to alter the net effect of the rotation ofthe eccentrics to vibrate the casing 52 of the tool head 41 so as todrive the drilling bar 40 downwardly or upwardly according to motordirection.

The principle of the drive system will now be explained with referenceto FIGS. 11a, 11b and 12a and 12b.

Consider first the case show in FIG. 11a, in which masses ml and m₂ arerotated with the same annular velocity ω, but in opposite directions,about respective axes P₁ and P₂. Then, there is a net force f generatedon the support zone at axis P₁ and P₂ which varies sinusoidally.

Next, consider the case where there are two further masses m₃ and m₄rotating respectively about the axis P₁ and P₂, with an angular velocityof ω₂ and in opposite directions. Then, as shown in FIG. 11b, a force f₂is generated which is again sinusoidal. The net force then depends onthe masses and angular velocities, which determines the phase of therotation of the masses.

Since forces f₁ and F₂ are both sinusoidal, it is evident that the netforce over a whole cycle will be zero. At first sight, therefore, theground penetration probe will not move. Hover, this does not take intoaccount the friction between the ground and the ground penetrationprobe, which must be overcome before any movement occurs. It is thenpossible to arrange for the maximum force to be either up or down, andsince the maximum force will then overcome friction by a greater amountthan other forces, this will impose a net movement on the groundpenetration probe.

In this embodiment of the present invention, axes P₁ and P₂ correspondto the shafts 63, and masses m₁ and m₂ are equal, as are masses m₃ andm₄. Moreover, assuming masses m₃ and m₄ are smaller, ω₂ is twice ω₁.

Suppose further that phases between the masses is chosen so that thereis a point in the cycle which masses m₁ and m₃ are simultaneouslydirectly above point P₁ and similarly masses m₂ and m4 are directlyabove point P₂. The resulting acceleration profile is then shown by thesolid line in FIG. 12. FIG. 12 shows that the upward acceleration has amaximum value which is greater than the downward acceleration at anytime of the cycle, although the time averaged acceleration over a wholecycle will be zero. Then, account must be taken of the friction thatwill exist between the ground penetration probe and the ground. Supposea net acceleration of x in FIG. 12 is needed to overcome that friction.It can then be seen from FIG. 12 that the upward acceleration issufficient to overcome the frictional resistance, so that the groundpenetration probe moves upwardly. The downward acceleration, however, isnever sufficient to overcome the friction and thus there is net movementof the ground penetration probe. Similarly, by arranging for the phasesof the masses to be such that masses m₁ and m₃ are aligned directlybelow the point P₁ and the masses m₂ and m₄ are directly aligned belowthe point P₂, the net acceleration then corresponds to the dotted linein FIG. 12. It can readily be appreciated that the downward accelerationovercomes the frictional force, thereby imposing movement on the groundpenetration probe, whereas the upward force does not.

Hence, by suitably controlling the phases between the masses, upward ordownward movement of the ground penetration probe can be achieved by theeffect of the rotating masses and their interaction with frictionalforces.

Referring again to FIG. 7 the head-up display 47 gives a visual displayof the relative positions of the ground penetration probe on theunderground object. The structure of that head-up display may correspondto that display produced by Fraser-Nash Technology Limited, in which animage is projected on to a semi-transparent spherical mirror so that thelight from the mirror is collimated and is thus perceived by the vieweras being at a distance.

The display that may be generated with the present invention is shown inFIG. 13. The display has three windows 80, 81 and 82. Window 81 is amagnified view of the image appearing in window 80, around the tip ofthe ground penetration probe. Each of the windows 80 and 81 display animage 83 of the ground penetration probe, a region 84 corresponding tothe ground, and an image 85 of the underground object. Thus, the usercan see the approach of the image 83 of the ground penetration probetowards the image 85 of the underground object, and can thereforecontrol the movement of the ground penetration probe 24 to ensureappropriate movement of the ground penetration probe 24 into the groundtowards the underground object 26. Hence, the user can see, in hisdirect line of sight, images corresponding to the separation of theground penetration probe 24 and the underground object 26. The image forthe window 80 may be generated using antennae 21 and 22, whereas theimage for the window 81 may be generated using the antennae 21 and 23.

The window 82 has a first region 86 which gives a plan view of the image85 of the underground object relative to the longitudinal axis of theground penetration probe, and also gives numerical data relating to theseparation of the ground penetration probe 24 and the underground object26.

In order for the region 84 in FIG. 13 corresponding to the ground tohave a surface which is horizontal, it is then useful for the probe tocontain a tilt sensor, since the information from that tilt sensor maythen be used in the generation of the display shown in FIG. 13. If theprobe is not maintained in a horizontal position, the information fromthe tilt sensor can then be used to incline the image 83 relative to theregion 84. This may be useful not only to give the user a warning thatthe probe is inclined, but also permits the user to steer the probe ifan inclined approach to the object is necessary, because of difficultyof access to the surface directly above the object. In the absence ofsuch a tilt sensor, the user must maintain the probe vertical, but thiscan be achieved by e.g. a spirit level on the probe itself. However, inthat case, the display shown in FIG. 3 will not respond to the tiltingof the probe and the boundary of region 84 corresponding to the surfacewill then always be perpendicular to the image 83, even if the probeitself is not vertical. This will not affect the display of the approachof the probe to the object, corresponding to the approach of the image83 towards the image 85, but the display of FIG. 13 will then give aless useful guide to the true position.

One possible problem which may arise when a ground penetration probe isdriven into the ground towards underground cable is that there may beother underground objects, such as other utility cables, in theimmediate vincity. If all are generating electromagnetic signals, theresulting signals detected by the antennae of the ground penetrationprobe will correspond to the composite signal detected, thereby givingan inaccurate measurement of the position of the underground object ofinterest. The fact that the measurement is faulty will be detectable bymoving the ground penetration probe a predetermined distance, or bytilting it, and comparing the predicted and measured locations of theobject, as previously described. However, although the operator willthen know that the underground object has not been located accurately,he would not be able to make an accurate location of it. However, ifeach underground object at the site to be investigated carriesalternating currents of different frequencies, the electromagneticsignals generated by each underground object will similarly be at adifferent frequency, and therefore resolvable by modulation at thelocator. Therefore, if the operator applies, by a suitable power source,alternating currents of different frequencies to each underground objectat the site to be investigated, the locator can then determine theseparation, in terms of both X and Y coordinates, of the locator andeach underground object. Moreover, since the positions of all theunderground objects are known relative to the locator, they are alsoknown relative to each other. Hence, the display shown in FIG. 13 maydisplay more than one underground object. The image of such a secondunderground object is shown at 87. Hence, by applying different signalsto the underground objects at the site to be investigated, the operatorof the ground penetration probe may be presented with a display showingthe position of all the objects at the site, so that the groundpenetration probe can be controlled so as to approach the undergroundobject of interest, and to avoid all the others.

As a further alternative, each underground object may carry an activemarker. Such active markers are known in themselves, and have atransponder tuned to a specific carrier frequency. When the transponderreceives a signal at that carrier frequency, the energy is converted byinduction in a tuned pick-up coil to power a re-transmitting circuit ofthe transponder to generate an output, that output being frequencymodulated at the carrier frequency, so that it carries encoding dataidentifying the active marker, and hence the object to which it isattached. If each underground object at a site carries such an activemarker, and each active marker is tuned to a different carrierfrequency, then the active marker of any one object can by triggered toidentify itself by the input of a signal at the carrier frequencygenerated e.g. by the locator itself. Hence, the locator can identifythe underground objects at the site, which again permits a visualdisplay similar to FIG. 13 to be generated.

Although the embodiment of FIGS. 7 to 13 made use of a drilling bar toform the ground penetration probe, the present invention is not limitedto use of such a drilling bar and other penetration probes such asblades may be used. Furthermore, although the driving of the groundpenetration probe by a motor-driven system has been illustrated, otherdrive arrangements may be used to generate a hammering or vibratingaction, such a pneumatic, hydraulic, or electric arrangement. Thedrilling bar may have a mechanical stop to limit the penetration thereofinto the ground to a predetermined depth. It may also have means forattaching a sleeve thereto, particularly when the ground is soft, toprevent collapse of soil and maintain visual access. Indeed, suitablevacuum extraction means 60 (see FIG. 7) may be provided to removematerial from the site of ground penetration, to clear a finer layer ofmaterial immediately adjacent the underground object.

FIG. 14 shows the use of a portable locator, being an alternative to thedrilling probe of FIG. 7. It comprises a locator 141 correspondinggenerally to the embodiment of FIG. 5, and a signal processing anddisplay unit 142, typically supported by shoulder and/or waist straps;they are shown separately in FIG. 15.

FIGS. 16 shows a typical form of display by display unit 142, showing avertical cross-section of ground and locator, with positional dataregarding separation of target line and locator.

While the foregoing has described the primary application of the locatorto subsurface objects, it may be applied equally to elongate conductorson or above the surface, e.g. to follow a guidance cable.

We claim:
 1. A locator comprising:a ground penetration probe having atleast two antennae therein with a predetermined separation therebetween,the antennae being arranged to detect electromagnetic signals from aconductor of an object; means for analyzing the electric signals todetermine the separation of the locator and object both in the direction(x) of the separation of the antennae and the direction (y) transversethereto; and means for controlling the drilling force of the roundpenetration probe depending on the separation of the ground penetrationprobe and the object.
 2. A locator according to claim 1, wherein theground penetration probe further comprises a removable sheath forreceiving said probe.
 3. A locator according to claim 2, having threeantennae.
 4. A locator according to claim 3, wherein the analyzing meansis arranged to analyze the electromagnetic signals detected by a firstand a second antenna of said three antennae when the locator and theobject have a separation greater than a predetermined value, and toanalyze the electromagnetic signals from the first and the third antennaof said three antennae when the locator and the object have a separationless than said predetermined value.
 5. A locator according to claim 4,having a transmitter for inducing signals into the conductor of theunderground object.
 6. A locator according to claim 5, having means forgenerating a display showing the spatial relationship of the probe andthe object, based on the electromagnetic signals.
 7. A locator accordingto claim 6, wherein the analyzing means is arranged to predict theelectromagnetic signals detectable by said antennae corresponding to adifferent separation of the locator and object.
 8. A locator accordingto claim 7, further including a tilt sensor.
 9. A locator according toclaim 8, wherein the analyzing means is arranged to compensate forperturbations or asymmetries in the electromagnetic signals indetermining the separation of the locator and the object.
 10. A locatoraccording to claim 9, wherein each antenna comprises at least twomutually perpendicular coils.
 11. A locator according to claim 10,wherein each antenna comprises three mutually perpendicular coils.
 12. Alocator according to claim 11, wherein the axes of the coils intersect.13. A locator according to claim 11, wherein the axes of any two coilsdo not intersect and the analyzing means is arranged to compensate forthe relative displacement of the coils in determining the separation ofthe locator and the object.
 14. The combination of a locator accordingto claim 13 and an object, the object having at least one active markerforming said conductor, said active marker being arranged to generate acoded signal, said locator having means for generating an input signalto said active marker for triggering said coded signal and means fordetecting said coded signal from said active marker.
 15. A locatoraccording to claim 6, wherein the means for generating the display is ahead-up display for an operator of the probe.
 16. The combination of alocator according to claim 15 and at least two objects, each objecthaving a conductor and each conductor generating electromagneticsignals, the electromagnetic signals of each conductor being different,wherein the means for generating a display is arranged to show thespatial relationship of the two objects.
 17. A method of locating anobject using a locator having at least two antennae, comprising thesteps of:(a) detecting electromagnetic signals from a conductor of theobject using said antennae when the locator is in a first position; (b)moving said locator from said first position to a second position; and(c) detecting the electromagnetic signals from the conductor of theobject using said antennae when the locator is in the second position;wherein:the electromagnetic signals from the conductor of the objectdetectable by said antennae when said locator is in the second positionare predicted on the basis of the electromagnetic signals detected inthe first position; and the predicted electromagnetic signals and theelectromagnetic signals detected when the locator is in the secondposition are compared.
 18. A method according to claim 17, wherein thelocator is tilted from the first position to the second position, andthe tilt is measured by a tilt sensor, and the measured tilt is used inthe predictions of the electromagnetic signals detectable by the locatorin the second position.